Test areas, generally formed on the surface of substrates are electrically connected to different parts of circuits in order to carry out checks on the correct operation of these circuits. The checks typically include measuring continuous control voltages at the test area and are carried out by means of microprobes. These microprobes are connected to suitable measurement equipment and are temporarily applied to contact points on the test area to carry out the measurements.
The known type of test areas for electronic circuits are in the form of metal contact points. These contact points, generally square in shape, have dimensions of the order of 20 .mu.m.times.20 .mu.m and are arranged on the surface of a substrate in which one or more electronic circuits are formed. The contact points are surrounded by an insulating layer called a passivation layer which forms a slight rounded mass in the surface of the substrate.
The contact points or pads can be connected to parts of an electronic circuit formed in the surface of the substrate, but also to parts of a circuit formed deep in the substrate. This is notably the case when the substrate is a multi-layer substrate comprising a stack of conducting layers separated by insulating layers. In this case, a contact point of a test area is formed in the surface of the substrate and is connected to a component of a deep conducting layer by means of conductor filled vias which pass through the insulating layers.
In order to carry out the control measurements on the electronic circuits, a tool fitted with one or more microprobes is brought to the surface of the substrate to a distance close enough for the ends of the microprobes to make contact with the substrate. The microprobes take the form of fine metal strips which are pressed onto the surface of the substrate. These fine metal strips are generally flexible and are comparable to hair.
A micrometric displacement device, such as a micropositioner, is then actuated to cause a relative displacement of the probes and the substrate along the plane of the substrate. This displacement is intended to bring the ends of the probes into coincidence with the contact points on the test areas. The relative displacement movement of the probes and the substrate can be controlled by various devices, such as optical location systems or viewing systems that use a CCD camera.
The main difficulty that is apparent during the control operation of the circuits described above is positioning the ends of the probes exactly on the contact points. Incorrect positioning of the probes can cause erroneous control measurements. Another difficulty is to keep the ends of the probes on the contact points. This is attributed to the fact that the contact points protrude from the surface of the substrate.
To facilitate the positioning and holding of the microprobes on the contact points, the dimensions of the contact points are commonly increased. However, increasing the dimensions, and hence the area of metal contact points on the surface of the substrate, is accompanied by the creation of or the increase of parasitic capacitances.
In effect, the contact points of the test areas define capacitors with the other conductive parts of the integrated circuits of the substrate. These capacitors have a capacitance that increases with the area of the contact points. These capacitances are parasitic capacitances which modify and impair the behavior of electronic circuits. This problem is a particular nuisance for electronic circuits operating at high frequency.
On the other hand, when the dimensions of the contact points are reduced so as to limit the parasitic capacitances, expensive equipment for adjusting the probes on the contact points must be installed and good electrical contact between the probes and the contact points is not always guaranteed. Furthermore, location of contact points of small dimensions on the surface of the substrate proves to be difficult.